The present invention relates to a demodulation system for use in digital communication and more particular, to a demodulation method based on π/4 shifted QPSK (π/4 shifted Quadrature Phase Shift Keying) modulation system or π/4 DQPSK (π/4 Differential Quadrature Phase Shift Keying) modulation system in digital communication and also to a demodulator therefor.
As a modulation system for use in digital communication, the π/4 shifted QPSK modulation is one of typical systems. The π/4 shifted QPSK modulation is a common system, in particular, in mobile communication. This is because, as one of its reasons, differential detection having an excellent resistance to propagation environmental fluctuations can be applied in its demodulation system. In this connection, the π/4 shifted quadrature phase shift keying modulation system is referred to also as the π/4 DQPSK modulation system. Thus these systems will be called π/4 shifted QPSK modulation or demodulation system, hereinafter for the sake of convenience of explanation.
JP-A-9-130442 entitled “π/4 shifted QPSK demodulator” discloses a demodulation system in a digital wireless receiver of a π/4 shifted QPSK demodulation system, which can compensate for a waveform distortion generated in a radio wave propagation link and can obtain a high quality of transmission. The prior art demodulator disclosed therein includes a synchronization detection circuit, a waveform equalizer and a differential demodulator, wherein the wave equalizer extracts a signal subjected to wave distortion compensation from an output of the synchronization detection circuit, and the compensated signal is differentially demodulated to obtain decoded data. With such an arrangement, the influences of the waveform distortion generated in the radio wave propagation link can be lightened and thus there can be realized a demodulation system which is excellent even in a signal-to-noise characteristic. However, similarly to other modulation systems, the π/4 shifted QPSK demodulator also requires use of a waveform equalizer having a very large hardware/software scale.
As has been explained above, the π/4 shifted QPSK modulation system is suitable for mobile communication. Similarly to other digital modulation systems, however, when there is a delay spread due to multiple propagation, intersymbol interference takes place, remarkably deteriorating its communication quality. The delay spread will be explained below.
FIG. 5 is an exemplary structure of a general prior art π/4 shifted QPSK demodulator. In FIG. 5, reference symbols 101-1 and 101-2 denote input terminals for input of a received signal, symbols 102-1 and 102-2 denote A/D converters, 103-1 and 103-2 denote filters, 104-1 and 104-2 denote samplers, 105 denotes a differential detector, 106 denotes a slicer (decider)/decoder, and 107 denotes an output terminal for output of a decoded signal. An I (in-phase) component and a Q (quadrature) component of a base-band converted received signal which is an input signal of the demodulator is converted at each of the A/D converters 102-1 and 102-2 are converted by the A/D converters 102-1 and 102-2 to respective digital signals. Thereafter, the digital signals are subjected at the filters 103-1 and 103-2 to removing operation of unnecessary components and to waveform shaping operation, and then subjected at the samplers 104-1 and 104-2 to extracting operation at symbol points to obtain symbol signals. The symbol signals are subjected at the differential detector 105 to differential detecting operation, symbol-decided at the slicer/decoder 106, converted to corresponding decoded bits, and then outputted from the output terminal 107.
Examples of waveforms of signals 108 and 109 in FIG. 5 are shown in FIGS. 6 and 7. In these examples, it is assumed that each received signal has a C/N (carrier to noise ratio) of 15 dB. FIG. 6 shows a differential detected input signal, in which an assembly or aggregation of 8 point symbols can be confirmed. FIG. 7 shows a corresponding differential detected output in which an assembly of four symbol points can be confirmed. Symbol decision is carried out based on each symbol is present in which one of four quadrants defined by real and imaginary axes on a complex plane, and the corresponding decoded bit is outputted. In this example, there is no bit error.
Next shown is an example wherein multiple propagation causes a delay spread. The example is a two-wave model as one example wherein a received signal contains a direct wave and a delayed wave component having a power ratio of −3 dB relative to the direct wave and a phase difference of 135 degrees and delayed by ¼ of a symbol time from the direct wave. FIG. 8 shows a differential-detected input signal in this model. It is difficult to identify a clear symbol assembly due to intersymbol interference caused by the delayed wave. FIG. 9 is a differential-detected output. From comparison of a waveform of FIG. 9 with a waveform of FIG. 7, it will be seen that a signal which goes beyond real and imaginary axes on a complex plane forming decision boundaries is present, thus generating a symbol error. In this example, a bit error rate is 2.0×10−2.
As in the example explained above, the delay spread causes deterioration of a communication quality. One of causes of the delay spread is multiple propagation of radio wave by reflections such as buildings or mountains. As one of methods for lightening the influence of the multiple propagation, it is conceivable to use an adaptive equalizer. Such adaptive equalizers include a linear equalizer, a decision feedback equalizer and a maximum likelihood sequence estimator (or a Viterbi equalizer as a derivative thereof). However, any of these equalizers requires complicated calculation, which results in that its hardware or software scale becomes very large.
As has been mentioned above, the prior art has problems which follow.
(1) In the presence of a delay spread, this causes a communication quality to be deteriorated.
(2) For the purpose of lightening the influence of the delay spread, an adaptive equalizer is required, which involves complicated calculation.
In particular, when an adaptive equalizer is employed for a delay spread as small as it causes somewhat deterioration of the communication quality, it is not necessarily suitable in many cases when consideration is paid to its cost or hardware size increased by the employment of the adaptive equalizer. Accordingly, for such a small delay spread, it is desirable to use a method for lightening the influence in a simple manner.